This invention relates to a thin-film magnetic head which is smaller than the conventional products in pole recession created in the magnetic film portion when the air-bearing surface is lapped. The invention also relates to a lap, a lapping liquor and a lapping method applicable to lapping of various types of products comprising composite material, which include not only the thin-film magnetic heads but also other articles having the portions differing in hardness or having a portion positively charged in the surface and a portion negatively charged in the surface when placed in an aqueous solution.
As discussed on page 28 of "Essentials of Working Techniques for Magnetic Heads" by Japan Industrial Technology Research Center (1985), cast iron, tin, stone, kemets and the like have been used for lapping of the magnetic heads. Especially for lapping of the air-bearing surface of the thin-film magnetic heads, tin has been popularly used as lap material.
Also, as mentioned on page 94 of "Foundation of Interfacial Phenomena" published by Asakura Shoten (1973), an anionic surfactant (sulfate of naphthalene condensate) has been generally used for preparing a lapping liquor having a non-polar powder such as diamond powder dispersed therein.
Lapping of the air-bearing surface of a thin-film magnetic head by use of a conventional lap such as mentioned above (especially one made of tin) had the problem that a pole recession 22 such as shown in FIG. 25 of the accompanying drawings is produced in the lapped air-bearing surface due to difference in hardness among substrate 13, protective film 12 and magnetic film 11 of the thin-film magnetic head.
Generation of said pole recession is ascribed to the variance in depth of indentation of the abrasive grains, which is caused due to difference in working efficiency for the different materials composing the magnetic head, said materials differing in hardness from each other, when the surfaces of said materials are subjected to finish lapping under a same lapping pressure. For example, in manufacture of a conventional thin-film magnetic head comprising a magnetic film having a Vickers hardness of about 200 kgf/mm and a protective film and a substrate both having a Vickers hardness of 1,300 kgf/mm or greater, there would be produced a pole recession of about 0.03 .mu.m at most in the magnetic film portion due to difference in hardness among said magnetic film, protective film and substrate.
It is known that when lapping is conducted on the air-bearing surface of a thin-film magnetic head by using a conventional lapping liquor such as mentioned above, an adsorption film may be formed on the protective film 12 (usually made of alumina) or on the magnetic film 11 and substrate 13 composing the magnetic head. (Whether such an adsorption film is formed or not depends,on the type (polarity) of the electric charges on the surface of the magnetic film 11, etc., and the type of the surfactant (either anionic or cationic) used in the lapping liquor). As a result of close investigation of the action of the adsorption film, the present inventors found that the portion where such adsorption film exists is harder to lap than other portion, and this is responsible for generation of pole recession 22 such as shown in FIG. 25. For example, when lapping is carried out by using a lapping liquor prepared by dispersing 0.6 g of diamond grains with an average grain size of 0.25 .mu.m in 500 ml of a 1.0 wt % aqueous solution of sodiumalkyldiphenyl ether disulfonate, there is produced a pole recession of about 0.03 .mu.m at smallest.
The presence of such a pole recession poses the problem that it becomes substantially impossible to reduce by such amount the flying height (indicated by 20 in FIG. 25) of the magnetic head, making it unable to reduce the recording wavelength as desired. There is a certain relationship between flying height of the magnetic head and recording wavelength. For instance, in the example shown in FIG. 26, when the flying height increases by 0.01 .mu.m, the bit length is accordingly increased by 0.03 .mu.m. Therefore, in the example shown in FIG. 26, when there exists a pole recession of 0.03 .mu.m in the air-bearing surface of the magnetic head, the bit length becomes 0.15 .mu.m longer than when no such pole recession exists, resulting in a corresponding decrease of recording density.